JPH0766039B2 - Thermal check method for electronic circuits using liquefied nitrogen - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a thermal check method for an electronic circuit, and more particularly to a thermal check using liquefied nitrogen for an electronic circuit.

[Prior Art] A thermal check is generally used to identify a deteriorated member in an electronic circuit. Large, complex circuits are typically integrated circuits mounted on multiple independent components and a "matrix" board (typically the components within the subsystem and the physical and electrical integrated printed circuit board). It consists of

In the use of such circuits in military, aerospace and other critical sectors, it is customary to perform various tests before assembling the entire device. The circuit described above undergoes a series of tests in which malfunctions are expected but not all defects can be identified.
It is possible to connect with very complex test equipment.

In such an example, to identify the defect location,
The circuit is thermal checked while connected to the test equipment. Regarding the defects such as minute cracks in the conductor that have undergone the thermal check, although the phenomenon has not been completely elucidated, it is thought that the connection will be released when cooled to -10 ° C level.

In this case, by temporarily correcting the defect, it is possible to perform an appropriate operation in the test apparatus, and thereby point out a member or a circuit region in which the defective portion is assumed to be present. Additional repairs or replacements of components or parts of circuits are being performed to repair damage.

Conventionally, various chlorofluorocarbons (CFCs) have been generally used as a refrigerant for a thermal check. The most common are the dichlorodifluoromethanes known as Freon 12 and Halon 22. CFCs are typically sprayed using aerosol cans (similar to hair spray cans) that workers can carry in a tool case.

[Problems to be solved by the invention] However, for several years, the influence of CFCs on the atmosphere has been a concern. First, the CFC molecule itself holds 20,000 times as much heat as carbon dioxide, causing a much larger greenhouse effect than would be expected from its concentration in the air. More importantly, chlorine is released when the CFC molecules decompose, which acts to destroy the ozone molecules.

Moreover, this destructive effect is repetitive and permanent, as each chlorine atom is repeatedly released and combines with new ozone molecules. It is well known that ozone molecules absorb most of the ultraviolet rays emitted by the sun, which are extremely harmful to all living organisms from the human body to unicellular organisms. Therefore,
Many governments around the world are now limiting the production and use of CFCs, with the goal of ultimately eliminating them.

For this reason, thermal check methods for circuits that replace CFCs have been searched. With heat pumps, it is difficult to cool to the desired temperature (ranging from -30 ° C to -60 ° C) within 5-6 minutes. Expanding the high-pressure gas can achieve temperatures as low as -30 ° C, but the high-pressure gas causes physical damage to the circuit under test. Thus, so far no suitable replacement method has been found.

Therefore, an object of the present invention is to provide a thermal check method for electronic circuits that does not use CFCs.

[Means for Solving the Problems] In order to solve the above problems, in a thermal check method of an electronic circuit having a step of spraying a coolant onto a surface of the present invention, liquefied nitrogen, at least a part of which is liquefied The surface is sprayed on the surface while keeping the phase, and nitrogen in the liquid phase is vaporized on the surface. Here, it is desirable that 30% to 90% of the molecular weight is sprayed as a liquid phase.

Also, a step of installing a closed probe having a thin thermally conductive wall portion such that the electronic circuit surface to be thermally checked and the outer wall of the thermally conductive wall portion are in contact,
In the step of spraying the liquefied nitrogen, the liquefied nitrogen may be sprayed through the inner wall of the wall surface of the probe.
At this time, it is desirable that the entire surface be a sufficiently flat surface.

Liquefied nitrogen may also be sprayed directly onto the surface of the electronic circuit. In this case, the liquefied nitrogen is preferably sprayed as a sufficiently parallel flow through the small circular opening, but it is also possible to spray it through the flat fan-shaped extended opening.

Furthermore, known cryosurgical instruments may be utilized in the step of drawing heat away from selected sites in the electronic circuit.

[Operation] Also, by spraying liquefied nitrogen, a direct parallel flow that is directly injected to the electronic circuit surface of a small area for thermal check is formed without affecting other surfaces, and the selected area is quickly cooled. To do. The parallel flow as described above is formed by the presence of liquid-phase nitrogen. The reason why the jet flows are parallel is not clear, but it may be related to the high surface tension of liquid nitrogen.

By vaporizing the liquefied nitrogen, the selected area of the electronic circuit surface of the small area where the thermal check is performed is quickly cooled. Further, by using the closed probe, the liquefied nitrogen sprayed inside the probe cools the contact surface of the probe with the electronic circuit surface, and a thermal check is performed.

Further, nitrogen is completely inactive and does not interact with each other with respect to the circuit material, and has only a cooling effect on the human body and environment without any other harmful effect.

[Examples] Details of the present invention will be described below with reference to preferred examples. In the present invention, liquid nitrogen containing 30 to 90% (molecular weight) of liquid phase nitrogen is injected onto the circuit surface in the step of thermal check of the integrated circuit.

Liquefied nitrogen may be sprayed directly onto the surface of the electronic circuit so that it is sprayed relatively parallel to a range of 10 inches or less in diameter. Further, it may be sprayed in a fan shape within a range of a length of 3/8 inch or more and a width of several 10 inches in the substrate.

In addition, a closed probe having a three-axis structure in which a gas vent pipe surrounds the liquid supply pipe and a vacuum heat insulation chamber surrounds the liquid vent pipe, or a simpler probe used for gas vent, It is also possible to spray liquefied nitrogen.

According to the invention, the relative amount of nitrogen in the liquid phase for spraying,
The injection amount, the opening portion, and the shape of the probe tip are all adjusted to be optimum as necessary.

In FIG. 1, liquefied nitrogen is injected through an opening 11 so as to form a sufficiently parallel stream 10. The distal end portion 12 in which the opening 11 is formed is screwed onto the pipe joint 13 of the liquid feed pipe 13, and the liquid feed pipe 14 is extended from the valve 15 and is operated by the handle 16. Is.

The valve 15 is mounted on a lid 17 screwed to the upper part of the dewar 18, and a liquid feed pipe (not shown) is extended from the valve 15 to the liquid in the dewar 18. The devices 11 to 18 are cryogenic liquid transfer units 19 of a known type. A typical dewar 18 is a stainless steel dewar that has a double-walled structure and is highly degassed between the walls to insulate internal nitrogen from the external environment.

The internal volume of the dewar may be approximately 1/3 liter, but it is more preferably 1/2 liter in consideration of weight and complexity of operation, capacity, stability of holding, and the like. Opening 11 is approximately 20 mil (0.5 mm) to 50
Set to a diameter of mil (1.2 mm).

The cryogenic liquid transfer unit 19 can have various shapes as long as the spraying of a predetermined amount of liquefied nitrogen containing a sufficient amount of nitrogen in the liquid phase (either from the opening or from the probe) is practically possible. Needless to say.

In FIG. 1, the nitrogen stream 10 is discharged onto the surface of the electronic member 22 placed on the circuit board 24. A member 26 (completely integrated circuit or the like) larger than the electronic member 22 is mounted on the circuit board 24. It should be noted that FIG. 1 is a schematic diagram for explaining the present invention in an easy-to-understand manner.

A wide range of fan-like sprays 28, as shown in FIG. 2, may be used to treat larger members having a further extension of member 26 of FIG. The spray 28 is sprayed from a relatively narrow and elongated opening 30 that flattens one end of a tubular structure 32 with a suitable fitting 34 that is easily attached to the fitting 13 of the cryogenic liquid transfer unit 19 of FIG. .

The tube diameter is preferably 1/4 to 3/4 inch to provide a length of the opening 30 of 1/2 to 1/4 inch. It is also possible to use a larger fan-shaped opening 30 depending on the amount of nitrogen that can be injected.

The use of liquefied nitrogen as a refrigerant for thermal checks has special advantages over the use of CFCs.

The aerosol-type can used to inject CFCs into the circuit produces a very widespread comb-like spray, making it impossible to locally spray CFCs to the smallest parts of the circuit.

On the other hand, in the case of liquefied nitrogen, the spray is sufficiently parallel, that is, does not spread radially. The reason for this is not clear, but it may be related to the high surface tension of liquefied nitrogen. In any case, when liquefied nitrogen is used, it becomes possible to locally spray liquefied nitrogen to a minimal portion of the circuit.

Therefore, from the nitrogen can, it is possible to perform spraying on an extremely limited area having the same diameter as the spray diameter of the spray flow and slightly wider than the diameter of the opening portion through which the spray flow is discharged. That is, it is possible to spray even a region having a diameter of 1 millimeter.

CFCs wet the sprayed surface and remain attached until they evaporate. Further, the CFCs are discharged to the surface of the member by the injection force (whether the injection force of the CFCs itself or another injection force). This often results in component surfaces that are larger than the desired test area being wetted by the CFCs and cooled.

On the other hand, a nitrogen flow of which at least a part is in the liquid phase (preferably having a high liquid phase ratio) collides with the member surface to define a wetting region, and within this region, liquefied nitrogen balls or drops are formed. Evaporates as it flows radially outward.

In this case, the wetted area is limited to a relatively small area. For example, a typical wet area is on the order of 10 cents or 5 cents. Liquefied nitrogen is radially ejected from the collision surface to the member surface having a small area, and evaporates into the atmosphere. CFCs, on the other hand, flow around the corners of the member and wet the other surfaces.

The minimum temperature of CFCs is -65% ℃ (typical spray temperature is higher temperature), therefore, the minimum effect of 1/4 cubic inch is enough to obtain the desired effect of locating defects. It takes more than 30 seconds to cool down. On the other hand, nitrogen is -19
It is possible to inject in the liquid phase at 6 ° C, and the member can be cooled more quickly. In fact, most parts can be cooled in 5 to 15 seconds with liquefied nitrogen.

In order to perform local cooling of a desired part with CFCs, a tool such as a cardboard dam that cools the cooled part more quickly without cooling other members is necessary. In order to diagnose and identify a defective portion, it is very important to maintain the accuracy of distinguishing the cooled portion and the uncooled portion of the circuit. The larger the cooled circuit area, the more difficult it is to pinpoint the exact damage area.

CFCs are easily scattered around and sometimes collide with the skin of workers. This is especially likely to occur when working with a container in one hand. Also, CFCs tend to adhere to the skin and moisten and cool them. On the other hand, when using liquefied nitrogen, it is not necessary to prevent momentary contact with the skin. Of course, prevention using gloves or other means may be performed if necessary.

CFCs chemically react with certain materials in electronic circuits,
Nitrogen is a substance that is completely inert to all the materials that make up electronic circuits.

CFCs are known to be toxic and hazardous to health in the workplace. The effects of CFCs on workers include dizziness, involuntary tremors, consciousness clouding, arrhythmias, and even death. It also has a high probability of causing frostbite on the skin and eyes. On the other hand, nitrogen is completely harmless in the amount used in the present invention, and does not harm the human body except that it takes away heat. The only effect on the human body is a slight increase in atmospheric nitrogen content (approximately 89%). Nitrogen, which is about the total intake of oxygen in the working environment of the worker, does not affect the human body at all. Further, as described above, nitrogen is safer to the skin than CFCs because it has a property of being scattered around as a liquid and vaporizing.

In addition, CFCs are known as relatively inexpensive substances, but by replacing them with nitrogen, the cost required for cooling can be reduced to 20 to 50% of that of CFCs.

The use of liquid nitrogen as a coolant for low temperature high speed supercomputers is well known. In such cases, the circuit is designed to be immersed in liquefied refrigerant,
The materials and other design components are selected from materials that are not structurally damaged at low temperatures and that do not change electronic phenomena except to increase circuit processing speed.

In circuit thermal checking, immersion in a cooling solution is not practical for the following reasons. That is, when the member is first immersed in the liquid, the liquefied nitrogen boils (vaporizes) violently due to the circuit structure that is warmer than the cooling liquid, and the nitrogen gas forms an insulating sheath between the liquid and the member. It disables the cooling of the circuit within seconds, which is an important factor for thermal checking. Moreover, cooling large areas of circuit components usually fails to provide a satisfactory indication of the defects required for thermal checking.

In addition, if members made of various materials (epoxy resin, etc.) are extremely cooled (-100 ° C or less), some materials tend to crystallize, which causes spontaneous cracking due to crushing or shrinkage. Is believed to cause.

On the other hand, since the liquefied nitrogen flow is controlled so that the minimum member of the circuit member is cooled, the cooling site is always limited to the surface of a single material, and crystallization and crushing can be prevented. In addition, the desired temperature (−20 to −60 ° C.) is sufficiently confined within a small area, so that defects can be pointed out in a relatively short time.

As described in the embodiment of FIGS. 1 and 2, liquefied nitrogen is injected directly onto the surface of the circuit under test,
Although superior to most of the techniques currently used with CFCs, a broader spray of liquefied nitrogen is expected in the thermal check technique.

At times, it may be more practical to use a closed probe for cooling rather than directly injecting liquid nitrogen onto the circuit surface under test. In such a case, it is possible to strongly cool the region adjusted very precisely by the closed probe, and the inner surface of the probe is cooled by direct contact collision of liquefied nitrogen with high liquefaction. ,
Meanwhile, the outer surface of the probe is attached to the surface of the check circuit.

FIG. 3 shows a part of the triaxial probe. The tip of the probe is composed of a tip 40 made of a material having a high thermal conductivity such as copper or silver. This tip has an outermost tube 42 and an inner tube 44, and a vacuum space is provided inside these.
Bonded coaxially to a vacuum insulation structure containing 46 in a metallurgical manner.

Inside the inner pipe 44, a nitrogen liquid feeding pipe 48 is installed so that the end is slightly separated from the tip portion 40. During operation, nitrogen is sent through tube 48, becomes fully liquid and collides with the inner surface of tip 40, partially vaporizing and vaporizing a considerable temperature from tip 40. Take away as heat. Nitrogen gas flows around the outer perimeter in space 50 between tubes 44 and 48. Such probes are commonly used in cryosurgery.

The problem with this probe is that the effect of removing heat from the surface due to contact between the surfaces is smaller than the effect of direct injection of liquefied nitrogen described above with reference to FIGS. 1 and 2.
This is due in part to the fact that these faces are not actually molecularly joined, so the face to be cooled is the cooled face (tip 40).
It is thought that this is because there are relatively few heat transfer paths.

It is also believed, in part, that a direct nitrogen stream of liquefied nitrogen across the surface is much more effective than cooling the surface. That is, it seems that cooling by the heat of vaporization at a low boiling point is more effective than simple introduction of heat into the substance at low temperature.

Another problem with using probe tips is that it is extremely difficult to mount the probe face in line with the device face to be cooled for maximum heat transfer between these surfaces. Therefore, it may be preferable to use liquefied nitrogen to spray properly directly onto the surface to be cooled, rather than using a probe.

However, if the principle of the present invention and the characteristics of nitrogen use are set to a wider range, there may be a spraying example in which use of a probe is considered desirable. That is, the present invention not only directly sprays liquefied nitrogen on the surface to be cooled,
It also includes spraying on the inner surface of the probe whose outer surface is in contact with the surface to be cooled.

The present invention further has the advantage that both the closed probe and parallel flow sprays described above are possible and nitrogen is inert to the material. For this reason, it is possible to use a surface that is not very close to the surface for cooling. In addition, even if the circuit is connected to a test device that uses a large number of clips, probes, etc., nitrogen can be easily controlled not only by the probe but also by the parallel flow at all of these connections,
It can also be sprayed by direct parallel injection.

It should be noted that the present invention is not limited to the embodiments disclosed for the purpose of explaining the invention, and all modifications, omissions and additions are possible without departing from the gist of the present invention described in the claims. .

[Effect] According to the present invention, since liquefied nitrogen is used for the thermal check of the electronic circuit, it is possible to inject directly in parallel to the circuit surface of a small area to be subjected to the thermal check without affecting the other surface.

It is also possible to bring the closed probe into contact with the circuit surface and spray liquefied nitrogen from the inside of the closed probe through the contact surface with the circuit to perform a thermal check.

In the present invention, not a man-made product such as CFCs, but a naturally occurring gas that is industrially easily used is used as a refrigerant, so that a raw material can be easily and inexpensively obtained.

Further, according to the present invention, the reaction temperature of the circuit can be cooled more quickly, and the refrigerant inert in the air is sprayed more accurately and with higher identification ability, so that it is harmless to the environment and the human body,
It is completely safe to use.

[Brief description of drawings]

In the accompanying drawings, FIG. 1 is a perspective view schematically showing a state in which liquefied nitrogen is directly injected onto a member of an electronic circuit, and FIG. 2 shows a fan-shaped injection opening used for a wide range of spraying. FIG. 3 is a perspective view schematically showing a probe that can be adhesively attached to the surface of an electronic circuit. 10 …… Parallel nitrogen flow 11 …… Opening 19 …… Cryogenic liquid transfer unit 22,26 …… Electronic components 24 …… Circuit board 28 …… Fan spray 30 …… Opening 40 …… Tip 42 …… Maximum Outer pipe 44 …… Inner pipe 48 …… Nitrogen liquid delivery pipe

Claims (7)

[Claims] 1. A thermal check method for an electronic circuit, comprising a step of spraying a coolant onto the surface of an electronic circuit, the method comprising spraying liquefied nitrogen onto the surface while leaving at least a part of the liquid nitrogen in the liquid phase. A thermal check method for an electronic circuit, wherein at least a part of liquid-phase nitrogen is vaporized on the surface. 2. The liquefied nitrogen sprayed on the surface has a molecular weight of
30% to 90% is a liquid phase.
The thermal check method described in the section. 3. The method further comprises the step of placing a closed probe having a thin thermally conductive wall portion such that an electronic circuit surface to be thermally checked and an outer wall of the thermally conductive wall portion are in contact with each other. The method according to claim 1, wherein in the step of spraying the liquefied nitrogen, the liquefied nitrogen is sprayed through an inner wall of the wall surface of the probe. 4. The method according to claim 3, wherein the entire surface is a sufficiently flat surface. 5. The method of claim 1, wherein the liquefied nitrogen is sprayed directly onto the surface of the electronic circuit. 6. The method according to claim 5, wherein the liquefied nitrogen is sprayed as a sufficiently parallel flow through a small circular opening. 7. The method according to claim 5, wherein the liquefied nitrogen is sprayed through a flat fan-shaped extension opening.